In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), organic photodetectors (OPDs), organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example of between 50 and 300 nm thickness.
One particular area of importance is organic photovoltaics (OPV). Conjugated polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies above 8%.
In order to obtain ideal solution-processible OSC molecules two basic features are essential, firstly a rigid π-conjugated core unit to form the backbone, and secondly a suitable functionality attached to the aromatic core unit in the OSC backbone. The former extends π-π overlaps, defines the primary energy levels of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), enables both charge injection and transport, and facilitates optical absorption. The latter further fine-tunes the energy levels and enables solubility and hence processibility of the materials as well as π-π interactions of the molecular backbones in the solid state.
A high degree of molecular planarity reduces the energetic disorder of OSC backbones and accordingly enhances charge carrier mobilities. Linearly fusing aromatic rings is an efficient way of achieving maximum planarity with extended π-π conjugation of OSC molecules. Accordingly, most of the known polymeric OSCs with high charge carrier mobilities are generally composed of fused ring aromatic systems and are semicrystalline in their solid states. On the other hand, such fused aromatic ring systems are often difficult to synthesize, and do also often show poor solubility in organic solvents, which renders their processing as thin films for use in OE devices more difficult.
Another particular area of importance are OFETs. The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are good processibility, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
In prior art, various materials have been proposed for use as organic semiconductors in OFETs, including small molecules like for example pentacene, and polymers like for example polyhexylthiophene. However, the materials and devices investigated so far do still have several drawbacks, and their properties, especially the processibility, charge-carrier mobility, on/off ratio and stability do still leave room for further improvement.
Presently the charge carrier mobility of OSC materials and their application in OFETs are the focus of many research groups. This is mainly driven by the tuneable electronic properties of these organic materials. These compounds are preferred alternatives to amorphous silicon technology due to their prospect of low-cost, solution processability, high throughput with low temperature deposition, the ease of large area device fabrication, light weight and flexibility.
A wide variety of organic compounds have been used to fabricate OFETs. A promising performance with enhanced electron delocalization, conductivity, ability to form planar conjugated structures has been observed for linearly fused polycyclic aromatic compounds. However, these materials so far do still have some drawbacks when used in devices like, for example, inadequate mobility to drive OLED displays, and potential stability issues.
The OSC materials as disclosed in prior art do thus still leave room for further improvement, for example regarding their processibility and their electronic properties. Therefore there is still a need for OSC materials for use in OE devices like OFETs and OPV cells, which have advantageous properties, in particular good processibility, high solubility in organic solvents, good structural organization and film-forming properties. In addition, the OSC materials should be easy to synthesize, especially by methods suitable for mass production. For use in OPV cells, the OSC materials should especially have a low bandgap, which enables improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, high stability and long lifetime. For use in OFETs the OSC materials should especially have high charge-carrier mobility, high on/off ratio in transistor devices, high oxidative stability and long lifetime.
It was an aim of the present invention to provide compounds for use as OSC materials that provide one or more of the above-mentioned advantageous properties, especially easy synthesis by methods suitable for mass production, good processibility, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing small molecules and conjugated polymers as disclosed and claimed hereinafter. These comprise polycyclic units with a difluorinated central benzene ring. It has been found that these fluorinated units, when used as monomeric units in organic pi-conjugated polymers, will deepen the HOMO level of the polymer and as such potentially improve the device stability.
In prior art various linearly fused polycyclic aromatic compounds have been proposed for use as organic semiconductors in OFETs, such as indacenodithiophene (IDT) as disclosed for example in WO 2010/020329 A1 and EP 2075274 A1, indacenothienothiophene (IDTT) as disclosed for example in WO 2013/010614 A2, indenofluorene (IF) as disclosed for example in WO 2013/124687 A1 and WO 2010/041687 A1, and their derivatives. WO 2010/041687 A1 discloses, among others, a compound of the following formula

However, fused polycyclic aromatic compounds with a fluorinated central benzene ring as disclosed and claimed hereinafter have hitherto not been disclosed or suggested in prior art.